The present invention relates to a method of correcting irregularities in the sensitivity of a line sensor for reading characters, images, and the like on an original document in a facsimile transmitter a plate-making image scanning apparatus, etc., and more particularly to a shading correcting method in an image reading device wherein an original document is irradiated by a light source such as a lamp, and light that has been reflected by or passed through the original document is led to a photoelectric transducer or line sensor for reading image information on the original document, the method comprising reading corrective data stored in a data memory for correcting the sensitivity of photoelectric transduction elements of the line sensor, and correcting an image signal from the light sensor with the read-out corrective data.
In printing and platemaking industries, image scanning recording/reproducing systems are widely used for electrically processing image information on original documents to produce film plates with a view to simplifying the operation process and increasing the image quality.
Such an image scanning recording/reproducing system is basically constructed of an image reading device and an image reproducing device. In the image reading device, image information on an original document which is fed in an auxiliary scanning direction is scanned by a line sensor in a main direction that is substantially normal to the auxiliary scanning direction, so that the image information can be converted to an electric signal by the light sensor. Then, the image information which has been photoelectrically converted by the image reading device is subject to various processes such as for gradation correction, profile emphasis, and the like dependent on platemaking conditions in the image reproducing device Thereafter, the image information thus processed is converted to a light signal represented by a laser beam, which is applied to an image recording carrier of a photosensitive material such a photographic film to record the image information thereon. The image recorded on the image recording carrier is then developed by an image developing device, and the developed image recording carrier is used as a film plate for printing operation.
The line sensor used for scanning the original document in the main scanning direction to read the image information in the image reading device, generally comprises a CCD (charge-coupled device) having an array of photoelectric transduction elements arranged in the main scanning direction. The photoelectric transduction elements of the CCD have different photoelectric conversion characteristics, respectively. Therefore, even when an original document of uniform density is scanned, the image signal produced by the line sensor may have irregular levels.
One proposed solution to the above problem is the generation of corrective data for correcting reading sensitivity irregularities of the photoelectric transduction elements or light sensitive cells (see Japanese Laid-Open Patent Publication No. 60-77575). FIG. 1 of the accompanying drawings shows such a proposed arrangement.
In FIG. 1, an object 10 from which an image is to be read, such as a reference sheet of paper, i.e., white paper, having a uniform reflecting surface, is irradiated with light from a light source 12, and light reflected from the reference sheet 10 is focused on the light detecting surface of a line sensor 16 having a linear array of photoelectric transduction elements by means of a focusing lens 14. The line sensor 16 is electronically scanned to produce an output which is stored element by element as corrective data in a corrective data memory 20 in preparation for shading correction.
Then, an actual original document is irradiated with light from the light source 12. An image read from the original document and represented by reflected light is focused on the light detecting surface of the line sensor 16 by the focusing lens 14. Then, the line sensor 16 is electronically scanned element by element, and the output from the line sensor 16 is converted to a digital signal by an A/D converter 18. The signal level of photoelectric transduction element is corrected in a correction processor 22 by the corresponding corrective data item stored in the corrective data memory 20 to obtain a reproduced image signal which has been corrected as to shading.
As shown in FIG. 2, the output from the line sensor 16 is saturated when the amount of exposure reaches and exceeds a certain level Therefore, the linear nature of the photoelectric conversion characteristic of the line sensor 16 is lost in the vicinity of the saturated value of the output from the line sensor 16. Curves b represent the photoelectric conversion characteristics of two different photoelectric transduction elements. Therefore, if the amount of exposure to the reflected light of the line sensor is near the saturated level when reading white paper or the like for shading correction, the corrective data stored in the corrective data memory is not appropriate, and no sufficient correction can be made in regions where exposure is low.